The present invention relates to a particulate ferrophosphorus composition having a surface deposit comprising mixed ions of chromium and a metal selected from the group consisting of zinc, strontium, calcium, barium, lead or copper. The surface deposit is sufficiently thin to provide the particles with enhanced conductivity and passivation resistance.
Coatings which are designed to provide cathodic corrosion protection for metal structures such as pipes, storage tanks, bridges, and the like, as well as metal surfaces used in marine applications, such as ship hulls, support structures for drilling rigs, docks, and the like, are well known in the art. The active component of such coatings generally forms an internal battery with the metal surface to be protected and thus act as sacrificial anodes. These coatings contain metal particles which are more anodic than the metal surface to be protected. The metal particles are present as a pigment in a suitable binder. Coating compositions utilize many organic or inorganic binder materials and a conductive metal such as particles of zinc.
In order to provide effective corrosion protection to the metal surface, appreciable amounts of zinc, i.e. 80% or more by weight of the total solids composition, are frequently used in the coating. Since zinc is a relatively expensive metal for this purpose, such large amounts of zinc add greatly to the cost of providing such protection.
Previous attempts to provide lower cost substitutes for zinc-rich coatings have focused on substituting other suitable metal pigment materials for at least a portion of the zinc in the coatings. The use of ferrophosphorus powder as a replacement for up to 50% or more of the zinc in these coatings is well established. See U.S. Pat. No. 3,562,124, issued Feb. 9, 1971. The resulting coating composition is not only less expensive than conventional zinc-rich coatings, but also, in some instances, possesses enhanced corrosion protection. Ferrophosphorus particles which are used in such applications are available commercially from the Occidental Chemical Corporation under the trademark Ferrophos.RTM. pigment.
The use of Ferrophos pigment in electromagnetic interference shielding (EMI) applications is also disclosed in the prior art. See U.S. Pat. No. 4,447,492, issued May 8, 1984 and U.S. Pat. No. 4,517,118, issued May 14, 1985. These patents disclose the use of Ferrophos alone or in combination with other conductive materials, such as nickel powder, in coating compositions which can be used to coat plastic substrates for electromagnetic interference shielding purposes. In this application, the coating serves to protect the substrate from electromagnetic energy emanating from other sources, or to prevent the escape of electromagnetic energy from within a housing formed from the plastic material. In order to be effective in this application, a pigment desirably has a high conductivity, i.e. 0.1 ohm-centimeter or less, as well as superior magnetic properties, and must be capable of providing attenuations of 30 decibles or more over a frequency range of 0.5 to 1,000 mhz.
Other applications for Ferrophos include electrostatic dissipation coatings and conductive coatings for use as components of electrical contacts, connectors, switches and printed circuit boards.
In all of these various applications, good electrical conductivity is essential and this will depend on good interparticle contact. An electrical current should flow from particle-to-particle with the lowest amount of contact resistance. Interparticle resistivity can increase, however, due to the formation of an oxide or other passivating film on the surface of the particle. Although relatively thin in terms of the dimensions of the particle, e.g. perhaps only a few atoms thick, these passivation layers impart a high resistivity to the surface of the particle and thus reduce the flow of current between contiguous particles. Attempts to clean the surface of the particles using, for instance, a dilute mineral acid solution are only temporarily successful since the passivation layer will readily reform on the particle surface. For this reason, most non-noble metals, such as copper and iron, are readily passivated and therefore fail in such applications. The failure rate is accelerated under conditions of elevated temperature and high humidity.
It will be readily appreciated, therefore, that a continuing need exists for inexpensive conductive pigments having improved characteristics such as electrical and magnetic properties, as well as improved resistance to passivation.